Magnetically actuated mechanical interlock



March 15, 1960 J. W. WURGLER MAGNETICALLY ACTUATED MECHANICAL INTERLOCK Filed May 9, 1956 2 Sheets-Sheet 1 June-kart 30%1 Q22 "awn/1W 24A 5 A WM J fukowvu 2 9 March 15, 1960 J, w, WURGLER MAGNETICALLY ACTUATED MECHANICAL INTERLOCK 2 Sheets-Sheet 2 Filed May 9, 1956 :armature movement to avoid such a condition. of protection using levers to amplify the armature movement which ofiers sufiicient protection is complex and :decreases system reliability.

United States Patent MAGNETICALLY ACTUATED MECHANICAL INTERLOCK John W. Wurgler, Wauwatosa, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

Application May 9, 1956, Serial No. 583,784

14 Claims. (Cl. 317-187) This invention relates in general to protective devices for electric machinery, and particularly to protective interlocks for electrical control systems using selectively operated relays.

Electromagnetic relays offer a convenient means of changing connections to an electric motor to obtain several different speeds or to reverse the direction of rotation. There are many advantages present in this type of control but there is a possibility that several relays may accidentally be actuated at the same time and improperly connect equipment to the power line. This is especially true in a motor speed control circuit where the motor has tapped windings or in a circuit to control the direction of motor rotation.

It is desirable to provide the protection as close to the relay as possible to avoid the possibility of a short developing in the lines feeding the electromagnet coils, and causing operation of the relay independent of the control switch.

One method of protecting the equipment from damage caused by simultaneous actuation of selective relays uses extra contacts on each relay. The electromagnet coil of one relay is connected in series with the extra contacts on the other relay. These extra contacts are closed only when the relay is deenergized to prevent simultaneous operation of both relays.

When two relays sequentially operated to reverse the direction of current fiow in a motor field it is desirable to prevent one relay from closing until the other is com- ;pletely open. An interlock which permits one relay to close when the other is partially opened does not protect against a short circuit through an arc across the partially opened contacts.

A system of levers may be used to amplify a small Amethod An object of this invention is to provide a simple, durable and inexpensive relay interlock which is entirely mechanical in operation does not require any additional electrical contacts.

Another object of this invention is to provide a simple mechanical interlock which permits only one relay armature to be operated at a time, thus affording protection to a circuit should several electromagnetic coils be energized simultaneously.

Still another object of this invention is to provide a mechanical interlock which will retain its protective features if it should become inoperative.

It is still another object of this invention to provide a mechanical interlock which is fully automatic in its oper ation and does not require an additional solenoid.

The accompanying drawings illustrate an embodiment of the invention which will now be described, it being understoodthat various modifications may be made in the embodiment illustrated without departing from the spirit and scope of the appended claims.

Fig. 1 is a front elevation view of two relays with the mechanical interlock of this invention.

Fig. 2 is the same view as Fig. 1 with the relay armatures and contact arm supports cut away to show the position of the member.

Fig. 3 is a side view of the relay assembly in the ener gized position with parts cut away.

Fig. 4 is the same view as Fig. 3 with the relay coil deenergized.

Fig. 5 is an isometric view of the relay assembly with parts removed, showing the position of the interlock member and the stop bracket in the energized position.

Fig. 6 is the same view as Fig. 5 with the relay coil deenergized and the interlock member in a different position.

Two similar relay assemblies 7 and 8 are mounted with bolts 9 passing through oversize holes in the relay mounting panel. The oversize holes in the relay mounting panel allow movement of the relays to provide relative adjustment of the relay pole pieces.

An interlock member 11 of magnetic material is slid-- ably positioned between the electromagnet pole pieces 12 and 13 of coils 14 and 15 by guide brackets 16 and 17. In the embodiment of this description the member is constructed largely of magnetic material. There is no reason why a large portion of the member could not be an insulating material or other suitable material. Contact arm supports 18 and 19 are fixed to the relay armatures 20 and 21.

Engagement means comprising stop brackets 22 and 23 are aflixed to the contact arm supports so positioned with relation to windows 24 and 25 in the interlock member 11 that tangs 26 and 27 of the stop brackets will pass into the windows as in relay 8, Figs. 1 and 2, or be blocked as in relay 7, Figs. 1 and 2, depending .on the position of the interlock member.

Contact carriers 28 hold conventional bridging contacts 29 which complete circuits through fixed contacts 30. A bias spring 31 adjusted by means including a screw 32, holds the armature away from the electromagnet pole piece against the armature stop 33 which is part of the guide bracket. An index slot 34 in the electromagnet core locates the guide bracket in a fixed relation to the electromagnet pole piece. 35 in the guide bracket restrain the interlock member to back and forth movement between the electromagnet pole pieces. Shims 36 between the stop bracket and the contact arm support allow adjustment of the gap between the interlock member and the tang. A facing 37 of nonmagnetic material prevents residual magnetism from hold ing the interlock member against the electromagnet pole piece after the solenoid is deenergized. Hole 38 in the guide bracket provides clearance for the stop bracket tang.

The invention may be more fully understood from the following description of the manner in which it functions.

At the beginning of an operation with both relays deenergized, the bias springs hold the relay armatures against the armature stop brackets biasing them away from their respective electromagnet pole pieces. The interlock member may be against either one of the electro magnet pole pieces or in some intermediate position. Although this member is made of magnetic material it is of relatively light construction and therefore slides' easily in guide slots provided in the mounting brackets.

A D.C. potential applied across the electromagnet solenoid causes an electric current to flow through the winding. Since any increase of current through the coil is Patented Mar. 15, 1960 The walls of guide slots.

value required to overcome the bias spring and attract the armature. As the current rises in the electromagnet solemold the flux density in the electromagnet core increases. This increasing flux density exerts a greater and greater attractive force on both the electromagnet armature and the interlock member.

The force required to move the bar need be sufiicient only to overcome friction, while the force exerted by the bias spring must be overcome to move the armature toward the pole piece. The magnetic fiux density in the electromagnet core required to position the interlock member against the pole piece depends upon the force required to overcome friction, and the air gap between the member and the pole piece in the separated position. In other words, a lighter member could be spaced further near the pole piece and a heavier member would necessitate a smaller air gap. A satisfactory weight and space relationship for the interlockmember is one in which the member is attractedto the electromagnet pole piece by a current approximately one-third of that required to attract the armature to the pole piece. This relationship insures that the interlock member will be attracted to the pole piece of the electromagnet and position itself before the relay armature begins to move toward the pole piece. After the flux caused by the current in the electromagnet coil increases to a point where it overcomes the force of the bias spring, the electromagnet armature moves toward the electromagnet pole piece.

As the relay armature moves toward the pole piece, the stop bracket tang passes through a window in the interlock member and allows the armature to travel the full distance to the pole piece. The clearance hold in the stop bracket insures that the stop bracket tang will pass completely through the window and allow the armature to rest against the pole piece. At this point the electromagnet relay armature has moved the movable bridging contacts against the fixed contacts and completed the circuit.

Should the other relay electromagnet now accidentally be energized, without deenergizing the first relay electromagnet, the flux build-up in the second electromagnet core would create a force attracting the interlock mem ber and the armature of the second relay toward the second pole piece. Since the interlock member is much closer to the first electromagnet core the sealing force of the flux already passing through this core into the interlock member is much greater than the attractive force exerted by the flux of the other electromagnet core.

..In. addition to this sealing force there is a latching action of the tang andwindow which prevents movement of the .interlockmember away from the pole piece when the armature rests against the pole piece. Thus it will be seen that the interlock member does not respond to the attractive force of the second relay electromagnet unless the first relay electromagnet is deenergized. Movement of the second relay armature toward the second relay pole piece is blocked by the action of the tang bearing against the interlock member, the tang and window being out of alignment unless the interlock member is positioned against the pole piece.

Should the first relay electromagnet be deenergized, and the second relay electromagnet be energized, the first armature is released from the first pole piece and the bias spring moves the armature away from the first pole piece. When the armature comes to rest against the stop bracket, the tang has been removed from the window in the interlock member and the interlock member is free to slide.

vToprevent residual magnetism in the interlock memher and the electromagnet core .of the first relay from holding the. mechanical interlock member against the core of the first relay,.a facing of nonmagnetic material has been afiixed to the surfaces of the interlock member that bear against the electromagnet pole pieces. 7

After the first tang has'clear'ed the window in the interlock member and the flux in the second relay electromagnet core has increased to a point sufficient to attract the interlock member to its core, the interlock member moves to the second pole piece. The second relay armature then moves down against the pole piece, passing its tang through a window in the mechanical interlock bar. The circuit controlled by the second relay is now energized and the first relay protected against accidental energization.

To keep production costs low and still maintain the close tolerances required in a system of this type, provision has been made for field adjustment of all critical dimensions. The distance between the pole pieces of the respective electromagnets determines the air gap between the mechanical interlock bar and the pole piece.

An adjustment of this distance is provided by the oversized holes in the mounting panel for the bolts holding the relays to the mounting panel. To make an adjustment the bolts holding the relay are slightly loosened and the air gap set by moving the relays toward each other or separating them as may be required to set the proper gap. The bolts are then tightened to retain the adjustment.

The clearance between the stop bracket tang and the interlock member may be adjusted by the use of shims placed between the stop bracket and the contact arm support.

Adjustment of the relation between the stop bracket tang and the interlock bar window is provided by oversize mounting holes in the stop bracket.

It can be seen that this mechanical interlock prevents the simultaneous energization of two circuits by either mechanical or electrical means.

Since the leakage flux oi the electromagnet solenoid armatures is utilized to move the interlock member, existing relays may be modified for interlock operation without substantial modification of the existing mechanism or the addition of electrical contacts.

A typical conversion would involve replacement of the existing armature stop bracket with the bracket of this invention containing the clearance hole for the stop bracket tang and the guide slots for the interlock member. The stop bracket is mounted on the armature with the same bolts that hold the contact carriers.

Although but one embodiment ofthe present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. A mechanical interlock comprising a plurality of electromagnets, each having a pole piece and amovable armature biased away from and coacting with said pole piece, interlock means comprising a movable member at least a part of which is of magnetic material, engagement means on said member and armature restraining movement of a first armature when said member is in a first position, said engagement means allowing movement of said first armature and restraint of said member when said member is in a second position, guide means positioning said member to be moved between the first and second position through alternate magnetic attraction of said member to said electromagnets.

2. A mechanical interlock comprising a plurality of electromagnets, each having a pole piece and a movable armature biased away from and coacting with said pole piece, interlock means comprising a movable member at least a part of which is of magnetic material, nonmagnetic material 'so'disposedbetwe'en said member and said --pole-pieces .to prevent, residuakmagnetism from holding said member to said pole pieces, engagement means on saidmemberand armature restraining movement of a first armature when said member is in a first position, said engagement means allowing movement of said first armature and restraint of said member when said member is in a second position, guide means positioning said member to be moved between the first and second position through alternate magnetic attraction of said member to said electromagnets,

3. A mechanical interlock comprising a plurality of relay electromagnets, each having a pole piece and a movable armature biased away from and coacting with said pole piece, a movable member at least a part of which is of magnetic material, tang and window combinations on said member and said armatures, said combinations restraining movement of a first armature when said member is in a first position and allowing move ment of the first armature and restraint of the member in a second position of said member, said member being moved from the first position to the second position through magnetic attraction of a first of said electromagnets, and from the second position to the first position through magnetic attraction of one other of said electromagnets.

4. A mechanical interlock comprising a plurality of relay electromagnets, each having a pole piece and a movable armature biased away from and coacting with said pole piece, a movable member at least a part of which is of magnetic material, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, tang and window combinations on said member and said armatures, said combinations restraining movement of a first armature when said member is in a first position and allowing movement of the first armature and restraint of the member in a second position of said member, said member being moved from the first position to the second position through magnetic attraction of a first of said electromagnets, and from the second position to the first position through magnetic attraction of one other of said electromagnets.

5. A mechanical interlock comprising a plurality of relay electromagnets, each having a pole piece and a movable armature biased away from and coacting with sid pole piece, a plurality of guide brackets with guide slots indexed to said electromagnets, a movable member at least a part of which is of magnetic material passing through said slots, tang and window combinations on said member and said armatures, said combinations restraining movement of a first armature when said member is in a first position and allowing movement of the first armature and restraint of the member in a second posi tion of said member, said member being moved from the first position to the second position through magnetic attraction by said first electromagnet, and from the second position to the first position through magnetic attraction of one other of said electromagnets.

6. A mechanical interlock comprising a plurality of relay electromagnets, each having a pole piece and a movable armature biased away from and coacting with said pole piece, a plurality of guide brackets with guide slots indexed to said electromagnets, a movable member at least a part of which is of magnetic material passing through said slots, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, tang and window combinations on said member and said armatures, said combinations restraining movement of a first armature when said member is in a first position and allowing movement of the first armature and restraint of the member in a second position of said member, said member being moved from the first position to the second position through magnetic attraction of said first electromagnet, and from the second position to the first position through magnetic attraction of one other of said electromagnets.

7. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to'the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so atfixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said Window, a first position of said member allowing movement of said tang into said window when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second step bracket tang so afiixed to a second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said memher from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic fiux of said second electromagnet, movement of said member from said second position to said first position caused by magnetic fiux of said first electromagnet.

8. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so affixed to a first electromagnet armature that said tang restrains movement of said armature to ward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so affixed to a second electromagnet armature that said second tang restrains movement of said second arma ture toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coactingwith walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, movement of said member from said second position to said first position caused by magnetic flux of said first electromagnet.

9. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so afiixed to a first electromagnetarmature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window, means providing an adjustment of the stop bracket tang position relative to the electromagnet armature to which it is afiixed, when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so affixed to a second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window,- a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, movement of said member from said second position to said first position caused by magnetic flux of said first electromagnet.

10. A mechanical interlock comprising first and see ond electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket in dexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so afiixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window, means providing an adjustment of the stop bracket tang position relative to the electromagnet armature to which it is affixed when said armature moves toward said pole piece, said tang when in said Window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so aflixed to a second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, move-- ment of said member from said second position to said first position caused by magnetic flux of said first electromagnet.

ll. A mechanical interlock comprising first and second electromagnets, each having a core with a .pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop'bracket tang so afiixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so afiixed to a second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, movement of said member from said second position to said first position caused by magnetic flux of said first electromagnet, said armature, said pole piece and said member so disposed that movement of said member from said second to said first position occurs before magnetic flux in said first electromagnet core has increased to a value sufficient to move said first armature toward said first pole piece and movement of said member from said first to said second position occurs before magnetic flux in said second electromagnet core has increased to a value sufficient to move said second armature toward said second pole piece.

12. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, walls of said guide slots serving to confined movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so affixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said members from movement away from said pole piece, a second window in said member, a second stop bracket tang so aflixed to a second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, movement of said member from said secnd position to said first position caused by magnetic flux of said first electromagnet, said armature, said pole piece and said member so disposed that movement of said member from said second to said first position occurs before magnetic flux in said first electromagnet core has increased to a value sufiicient to move said first armature toward said first pole piece and movement of said member from said first to said second position occurs before magnetic fiux in said second electromagnet core has increased to a value sufficient to move said second armature toward said second pole piece.

13. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece, movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at least a part of which is of magnetic material, Walls of said guide slots serving to confine movement of said member to a sliding motion between said pole pieces, a first window in said member, a first stop bracket tang so afiixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allow-,

ing movement of said tang into said window, means providing an adjustment of the stop bracket tang position relative to the electromagnet armature to which it is affixed when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so affixed to a'second electromagnet armature that said second tang restrains movement of said second armature toward a second pole piece unless said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said secondtposition caused by magnetic flux of said second electromagnet, movement of said member from said second position to said first position caused by mag- ,netic flux of said first electromagnet, said armature, said pole piece and said member so disposed that movement of said member from said second tosaid first position occurs before magnetic flux in said first electromagnet core hasincreased to a value sufficient to move said first armature toward said first pole piece and movement of said member from said first to said second position occurs before magnetic flux in said second electromagnet core has increased to a value sufiicient to move said second armature toward said second pole piece.

14. A mechanical interlock comprising first and second electromagnets, each having a core with a pole piece. movable armatures biased away from and coacting with said pole pieces, a first guide bracket indexed to the first electromagnet core, a second guide bracket indexed to the second electromagnet core, guide slots in said guide brackets, a movable member at leas't'a part of which is of magnetic material, nonmagnetic material so disposed between said member and said pole pieces to prevent residual magnetism from holding said member to said pole pieces, walls of said guide slots serving to confine movement of said member to a sliding-motion between said pole pieces, a first window in said member, a first stop bracket tang so afiixed to a first electromagnet armature that said tang restrains movement of said armature toward a first pole piece unless said tang may pass into said window, a first position of said member allowing movement of said tang into said window, means providing an adjustment of the stop bracket tang position relative to the electromagnet armature to which it is affixed when said armature moves toward said pole piece, said tang when in said window coacting with walls of said window to restrain said member from movement away from said pole piece, a second window in said member, a second stop bracket tang so afiixed to a second electrornagnet armature that said second tang restrains -movement of said second armature toward a second pole piece unless :said second tang may pass into said second window, a second position of said member permitting movement of said second tang into said second window 'when said second armature moves toward said second pole piece, said second tang when in said second window coacting with walls of said second window to restrain said member from movement away from said second pole piece, movement of said member from said first position to said second position caused by magnetic flux of said second electromagnet, movement of said member from said second position to said first position caused by magnetic flux of said first electromagnet, said armature, said pole piece and said member so disposed that movement of said member from said second to position occurs before magnetic flux in said first electromagnet core has increased to a value sufiicient to move said first armature toward said first pole piece and movement of said member from said first to saidsecond posi tion occurs before magnetic flux in said second electromagnet core has increased to a value sufiicient to move said second armature toward said second pole piece.

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